1. Field of the Invention
The present invention relates to electronic equipment such as an optical disk device or a rewritable CD-ROM device, which performs writing and reading operation by using a laser diode as a light source. More particularly, it relates to an auto power control circuit for a laser diode or an auto laser power control (ALPC) circuit which is used to control a power to be supplied to the laser diode to keep the optical output from the laser diode constant.
2. Description of the Related Art
An optical disk device or a rewritable CD-ROM device uses a laser diode as a light source, and performs data writing and reading operation by irradiation of a laser light onto a disk. However, the optical output from the laser diode varies widely according to ambient the temperature, the operating period of time and so on. As is understood from FIG. 6, for example, even if the laser diode is driven by a certain current IF, the optical output power Po significantly varies according to the temperatures Tc of 50xc2x0 C., 25xc2x0 C., 0xc2x0 C. and xe2x88x9225xc2x0 C. It may thus occur that the oscillating operation of the laser diode stops or the optical output from the laser diode becomes too large with result of destruction. It is therefore required to control the driving current to be supplied to the laser diode in order to obtaing a substantially constant optical output power.
To this end, the ALPC circuit is provided to detect the optical output from the laser diode and to then control the driving current flowing through the laser diode such that the optical output from the laser diode is kept constant.
The description will be made on the fundamental ALPC circuit with reference to FIG. 7. This ALPC circuit 100 has a laser diode (LD) 1, a photodiode (PD) 2, a current-to-voltage converter or an I/V converter 3, an operational amplifier 4 and a current booster 5. The photodiode 2 is used to detect the optical output 101 from the LD1 and thus generates a detected current Is that is representative of the power of the optical output 101. This current Is flows through a resistor RM in the I/V converter 3 so that an optical detection voltage Vd corresponding to the optical output detection current Is is generated across the resistor RM. This voltage Vd is supplied to an inverting input terminal (xe2x88x92) of the operational amplifier 4 having a non-inverting input terminal (+) supplied with a reference voltage Vref. The current booster 5 is constituted by a PNP transistor Q and an resistor RL connected between the emitter thereof and a power voltage line Vcc. The base of the transistor Q is connected to the output of the operational amplifier 4, and the collector thereof is connected to LD1 to supply a driving current IF thereto.
In this manner, the driving current IF of the LD1 is controlled such that the optical detection voltage Vd becomes equal to the reference voltage Vref. The optical output from the LD1 is thus controlled to be constant. For example, when the optical output 101 from the LD1 increases due to the temperature variations, the optical detection current Is from the photodiode PD2 increases accordingly. The increase in the optical detection current Is makes larger the voltage drop across the resistor RM to lower the optical detection voltage Vd. In response thereto, the operational amplifier 4 increases the base potential of the transistor Q, so that the driving current IF is made small. The optical output power 101 of the LD1 is thus decreased.
Based on the above ALPC circuit 1001 an optical disk device according to the prior art is equipped with an ALPC circuit 100 as shown in FIG. 8. It is to be noted that in the optical disk device, the required optical output from a laser diode differs according to the operation modes such as a write operation mode, an erase operation mode and a read operation mode. Therefore, the auto laser power control circuit 1000 is provided with a WRITE block 10, an ERASE block 20 and a READ block 30, one of which is brought into an active state in accordance with the operation mode to be currently initiated to control the driving current of the laser diode (LD) 1 during each operation mode. These blocks are substantially identical in configuration with one another. Accordingly, a description will given only to the WRITE block 107. It is noted that the same constituents as those shown in FIG. 7 are indicated by the same reference numerals to omit further description thereof.
The current flowing through PD2 in response to the optical output from the LD1 driven by the current booster 5 is supplied to the WRITE, ERASE and READ blocks 10, 20 and 30, each of which thus includes the I/V converter 3. The conversion voltage V1 is supplied through a resistor R1 to the operational amplifier 4, differently from FIG. 7. The resistor R1 determines the gain of the operational amplifier 4 together with a resistor R2 connected between the output and the non-inverting terminal of the operational amplifier 4. Such gain is set to be a considerable value, 100 for example, because a high sensitivity is required for this kind of device. However, such high gain then may sometime cause undesirable overshoot and/or undershoot in the output of the operational amplifier 4. A capacitor C is therefore connected in parallel to the resistor R2, thereby solving such a problem.
As is further distinct from FIG. 7, the reference voltage V2 to be supplied to the amplifier 4 is derived from digital data. Specifically, the reference voltage digital data WRCUR is supplied from a system controller (not shown) in the write operation mode and is converted into a reference voltage V2 by a D/A converter 6, which the voltage is then supplied to the amplifier 4 as a reference voltage Vref shown in FIG. 7 through a switch SW0 provided between the D/A converter 6 and the operational amplifier 4. The switch SW0 is controlled by a write operation mode signal C0 that assumes an active level in the write operation mode and an inactive level in the other modes. The active level of the signal C0 causes the switch SW0 to select the voltage converted by the D/A converter 6 and supplies it to the operational amplifier 4 as the reference voltage V2. When the signal C0 indicates a mode other than the write operation mode, on the other hand, the switch SW0 selects and supplies the ground potential to the operation amplifier 4, so that the LD1 is maintained uncontrolled from the WRITE block 10 even if the malfunction of the current booster 5 occurs. As is readily understood from the foregoing, each of the other blocks 20 and 30 is activated by the corresponding signal to the control signal C0 in the same manner with the unique digital data for reference voltage to corresponding mode. Each of the outputs WLD, ELD and RLD of the blocks 10, 20 and 30 is then supplied to the current booster 5. Although not shown, the current booster 5 is constructed to one of the signals WLD, ELD and RLD in response to the operation mode to be currently executed.
Thus, the LD 101 is controlled to output a laser with a substantially constant power in the respective operation modes.
It has been, however, recognized by the inventor that the ALPC circuit 100 has the problem that the shift in operation from one mode to another mode to be a relatively long period of time to deteriorate a high speed operation. This problem becomes remarkable upon the operation being moved from the erase or read mode to the write mode. This will be described below in details with reference to FIG. 9 which shows the signal voltage waveforms of respective parts in the WRITE block 10 in case where the operation mode is shifted from write to read, and then back to write.
In the write operation mode shown on the left-hand side of FIG. 9, the signal C0 assumes a high level as an active level, so that the reference voltage V2 based on the data WRCUR is supplied to the operational amplifier 4. Thus, the output terminal WLD voltage of the block 10 is controlled such that the conversion voltage V1 becomes equal to this voltage. As a result, the optical output from the LD1 is kept constant.
By the selection of the read operation mode, the signal C0 is changed to a low level as an inactive level. As a result, the ground potential is supplied to the operational amplifier 4 through the switch SW0, so that the output from the operational amplifier 4, i.e., the voltage of the WLD terminal is also changed to the ground potential.
On the other hand, the current booster 5 selects the output voltage of the READ block 30. As a result, the LD1 is held under the control of the READ block 30. Thus, the current from the PD2 becomes the optical detection current in the read operation mode. Accordingly, the signal voltage V1 in the WRITE block 10 becomes the voltage value corresponding to the data RECUR in the READ block 30. Herein, in the optical disk device, the driving current of the LD1 required in the write operation mode is considerably larger than that in the other mode. Therefore, the voltage based on the RECUR is considerably smaller as compared with the voltage based on the WRCUR.
When the read operation has been completed, and the write operation mode is selected again, the signal C0 is changed to the high level. As a result, the switch SW0 selects the D/A converter 6, so that the voltage V2 rises to the voltage value corresponding to the WRCUR. However, the voltage of the WLD terminal does not follow the voltage V2, but rises with a certain time constant as shown in FIG. 9.
Specifically, as described above, the capacitor C is provided in order to suppress the overshoot and the undershoot of the signal WRCUR which would be otherwise caused by the high gain, about 100, of the operational amplifier 4. For such a purpose, the capacitance value of the capacitor C is required to be set large (0.01 to 0.1 xcexcF). When the write operation mode is selected, the operational amplifier 4 is first required to recharge the capacitor C which has been discharged in the previous read operation mode. However, the capacitor C has the above large capacitance value, and hence the charging of the capacitor C takes as much as several tens of xcexcsec. The conversion voltage V1 also changes gradually. During this period, the optical output level from the laser diode LD1 is of course not stabilized. If information is written on a disk in such unstable state, the writing accuracy is deteriorated. At the worst, erroneous information may be written. For this reason, the actual write operation has to be initiated after elapse of such unstable period. Thus, the shift in operation mode cannot attained at a high speed.
As described above, a quick shift from the operation mode (ex., read operation mode) requiring a small driving current for the laser diode LD1 to the operation mode (write operation mode) requiring a large driving current is not executed. Further, there is also a high risk of erroneous writing.
An object of the present invention is to provide an improved ALPC circuit.
Another object of the present invention is to provide an ALPC circuit capable of quickly executing a transition from one operation mode to another operation mode.
A still other object of the present invention is to provide an ALPC circuit whereby the response time of a laser diode to the change in operation mode is improved.
A still further object of the present invention is to provide an ALPC circuit capable of improving the writing accuracy by a laser diode by providing a higher-speed output response upon the switching of the operation mode, while stabilizing the loop operation in the stationary state.
An ALPC circuit in accordance with the present invention is constructed such that a voltage responsive to the optical output of a laser diode generated in accordance with a driving current flowing therethrough is compared with a reference voltage to produce a voltage difference, and the driving current is controlled so as to decrease the voltage difference with a first time constant (or first driving ability) during a steady operation and with a second time constant (or second driving ability) that is smaller than the first time constant (the first driving ability) upon initiation.
More specifically, controlling the driving current during the steady operation is executed with such a first time constant (ability) that suppresses an undesirable overshoot and/or undershoot. On the other hand, upon initiation in which a transition from a first mode such as a read operation to a second operation mode such as a write operation occurs, the control to the driving current is not executed with the first time constant (ability), but is executed is with a second, smaller time constant smaller (that is, a second, larger driving ability) than the first one. In this manner, the operation mode is quickly shifted, and the stabilization in the optical output of the laser diode is also improved.
The period of time during which the driving current is being controlled with the second time constant (second driving ability) may be determined by observing a signal based on the optical output of the laser diode. However, it is preferable to set such period of time at a certain balue without observing the signal derived from the laser diode. This control may be attained in a timer operation manner. This is because that the second time constant (second driving ability) may be easily obtained by deactivating the feedback loop that functions during the steady operation and such deactivation can be attained by the state of a switch that is controlled by a timer.
In more detail, the control of the driving current during the steady operation is performed by a operational amplifier that compares the voltage indicative of the optical output of the laser diode with the reference voltage, and the parallel connection of a resistor and a capacitor is provided between the input and output terminals of the operational amplifier to control its gain and to suppress the undesirable overshoot and/or undershoot. In such configuration, therefore, a switch is provided in parallel to the parallel connection in accordance of the present invention. This switch is brought into an OFF state during the steady operation and into an ON state upon initiation. The state of the switch is changed to the OFF state after a predetermined period of time that is controlled by a timer. It is thus possible to obtain the first and second time constants (first and second driving ability).
An auto laser power control circuit according to another aspect of the present invention includes an operational amplifier producing an output signal in response to a voltage difference between a voltage representative of a laser power of a laser diode and a reference voltage and a driving circuit driving the laser diode in response to the output so as to make the voltage difference small, with the amplifier changing its output signal at a first rate during a predetermined period of time from a time point at which the auto laser power control circuit is initiated and thereafter changing the output signal at a second rate that is lower than the first rate.